Looking at Davis spec sheets I’ve found something interesting: a linear relation between solar sensor output and solar radiation. Exactly 1.67 mV per each W/m2.
What about Labjack and 1-Wire outputs? Not sure on 1-Wire, but my Labjack unit reads mV as well from my calibrated solar cell. But… does WD make this linear relation? I don’t think so. Seems to me that WD makes a linear relation, not related to W/m2 but to solar percent. And this is an error in concept.
Imagine June 21, full sun. I read my sensor output, say 250 mV. So WD thinks that 250 mV is equal to 100% solar: Ok. But today I read my solar sensor output at noon, full sun too. I read 125 mV. WD thinks it is 50% solar: ERROR. It is 100% possible solar as well, but the energy is lower than in June.
What it is: if 250 mV is 100% (say 1200 w/m2 at my site), then 125 mV is 50% (600 w/m2 at my site).
What it should be: If 250 mV is 1200 w/m2, then 1 w/m2 is 0.20 mV, ant thus 125 mV is 125 / 0.20 = 625 w/m2
Should it be better to match max output with max energy? Can any 1-Wire solar user take a look at this?
Now that we are closer to winter solstice, my Davis readings on a sunny day reach 100% (right), Labjack ones hardly reach up to 65%.
But is your labjack/cell set up linear? The Davis is because that’s the way the circuit is designed. I don’t recall what the setup is, but if you are just using a resistor load for the cell then it will not be perfectly linear, and it could be very non-linear.
I really doubt that WD is coded to convert labjack millivolts to solar % and not w/m2, that wouldn’t make any sense. You could test that easily though, just feed a constant x millivolts to the labjack and see how/if the solar reading varies during the day.
I really doubt that WD is coded to convert labjack millivolts to solar % and not w/m2, that wouldn't make any sense. You could test that easily though, just feed a constant x millivolts to the labjack and see how/if the solar reading varies during the day.
Good idea, I’ll test it that way on weekend. Thanks Niko.